The earliest known eutherian mammal

Abstract

The skeleton of a eutherian (placental) mammal has been discovered from the Lower Cretaceous Yixian Formation of northeastern China. We estimate its age to be about 125 million years (Myr), extending the date of the oldest eutherian records with skull and skeleton by about 40 50 Myr. Our analyses place the new fossil at the root of the eutherian tree and among the four other known Early Cretaceous eutherians, and suggest an earlier and greater diversification of stem eutherians that occurred well before the molecular estimate for the diversification of extant placental superorders (104 64 Myr). The new eutherian has limb and foot features that are known only from scansorial (climbing) and arboreal (tree-living) extant mammals, in contrast to the terrestrial or cursorial (running) features of other Cretaceous eutherians. This suggests that the earliest eutherian lineages developed different locomotory adaptations, facilitating their spread to diverse niches in the Cretaceous.

A new Early Cretaceous eutherian mammal from the Sasayama Group, Hyogo, Japan

“…The angular process of the dentary of S. kawaii is slightly inflected medially, which is a character seen in most metatherians including Cretaceous members [11,32]. However, an inflected angular process is also present in some early eutherians such as Eomaia, Montanalestes and Prokennalestes [4,14,33], and the angular process in these taxa is also rod-like, being morphologically similar to that of S. kawaii. Compared with the angular process of the basal metatherian Deltatheridium, the rod-like angular process of S. kawaii is clearly different from the shelf-like process of Deltatheridium [34].…”

“…This indicates that the absence of the entoconid in S. kawaii can be interpreted as a primitive condition. However, the lower molars of Jurassic-Cretaceous eutherians known thus far including those of taxa older than S. kawaii, such as in Acristatherium, Eomaia, Juramaia and Murtoilestes, possess distinct entoconids [3][4][5]38]. Ausktribosphenids and henosferids also have entoconids [39][40][41][42], whereas Ambondro does not have a distinct entoconid [43].…”

“…The primitive eutherian premolar count is generally considered to be five [12,13], as shown by early eutherian taxa, such as Acristatherium, Eomaia and Juramaia [3][4][5]. Other Early Cretaceous eutherians with known premolar counts (Prokennalestes and unnamed eutherians from the Lower Cretaceous Shahai and Fuxin formations of northeastern China) also have five premolars [14][15][16].…”

Mitogenomic Analyses Place the Gharial (Gavialis gangeticus) on the Crocodile Tree and Provide Pre-K/T Divergence Times for Most Crocodilians

“…The split between South American and Australian marsupials is not expected to be older than 70 MYBP or younger than 60 MYBP (Nilsson et al 2004). The divergence time between rodents and artiodactyls cannot be older than Eomaia, the oldest eutherian (Ji et al 2002), or younger than 84 MYBP, as eutherians began to diversify around this time (Archibald et al 2001).…”

“…These dates are consistent with previous estimates and the fossil record. The youngest eutherian fossil (Eomaia) is about 125 myr old (Ji et al 2002), suggesting a lower bound of about 130 MYBP for the split between marsupials and eutherians. An upper bound of 170 MYBP for this split is indicated by some molecular estimates (e.g., Kumar and Hedges 1998).…”